1. Field of the Invention
This invention relates to a modulating device for a modulation chain of the single-sideband (SSB) type.
SSB modulation consists in amplitude-modulating a carrier wave delivered by a local oscillator and in carrying out filtering of the modulated signal thus obtained with a view to retaining only one sideband of the frequency spectrum of the modulated signal. A distinction is drawn between lower SSB modulation which retains the modulated signal having a lower frequency than the carrier-wave frequency, and upper SSB modulation which retains the modulated signal having a frequency which is higher than the carrier-wave frequency.
This method of modulation offers an advantage over conventional amplitude modulation and at equal transmitter power in that the signal-to-noise ratio is improved at the transmission end by a quantity equal to 9 dB. This gain is a well-known result of SSB modulation and is the feature which makes the method attractive.
When a limitation is imposed on the transmitter power output (technical limitation in long-range transmitters or administrative limitation in configurations relating to allocation of the different frequency ranges), it is still possible to improve the signal-to-noise ratio by modifying the actual shape of the modulating signal.
Especially in the case of speech, the modulating signal in fact exhibits a substantial amplitude variation of the order of approximately 10 dB. This is caused by the presence of plosive syllables which usually begin with letters such as b, p, or t, and this holds true even in the case of a speaker who has a uniform tone of voice. The nominal transmission power of a transmitter is therefore assigned to the transmission of these signals of maximum strength. In point of fact, however, these signals represent only a small proportion of the quantities of information to be transmitted and are not entirely necessary for intelligibility of a message. Let P.sub.max be the peak power developed by a transmitter in order to transmit these high-amplitude signals and let P.sub.mean be the mean value of the power developed in order to transmit the greater part of the messages. It is known in this case that the ratio of useful signal to noise is equal to P.sub.mean /P.sub.noise and that the nominal power of the transmitter is equal to the peak value P.sub.max. It may thus be readily deduced that the power transmitted practically continuously by the transmitter is lower than its nominal power and that the signal-to-noise ratio can be improved in the proportion P.sub.max /P.sub.mean.
2. Description of the Prior Art
A first solution conceived in the prior art for reducing this variation in vocal amplitude consisted in clipping the modulating signal. This produced a rapid limitation, however, as a result of losses in intelligibility arising from distortions. A second solution proposed in the prior art consisted in regulating the gain of the SSB transmission system by means of an automatic device known as a compressor and in giving said compressor a time constant of the order of 20 to 50 ms. Under these conditions, consideration was not given to the variation in amplitude due to the plosive syllables (not even those which had the lowest frequency) but was given solely to the different intonations or tones of voice adopted by a speaker. This device permitted an improvement in the signal-to-noise ratio of approximately 6 dB.
The most effective method which permits a maximum increase in the mean power and therefore in the signal-to-noise ratio without any appreciable impairment of intelligibility consists in clipping the SSB signal by reason of the fact that, in SSB transmission, part of the quantity of information to be transmitted is conveyed by means of a phase modulation of the carrier wave. This can be explained as follows: let .omega..sub.p be the angular frequency of the carrier wave and let .omega..sub.m be the angular frequency of the modulating signal; the value of the modulated signal in SSB transmission is m cos (.omega..sub.p .+-..omega..sub.m) t, depending on whether a higher or lower SSB modulation is performed. Peak-limiting or clipping in SSB transmission consists in modifying m when m is too high by reason of a modulating signal of excessive strength. After clipping, said signal is certainly m' cos (.omega..sub.p .+-..omega..sub.m) t, with m'&gt;m, but nevertheless contains the information .omega..sub.m which is capable of ensuring intelligibility of the message. Under these conditions, the mean transmission power can be closer in value to the nominal power of the transmitter and the signal-to-noise ratio can come as close as possible to its maximum value.
In the prior art, the above-mentioned SSB peak-limiting operation performed after SSB filtering entailed the need for complementary filtering in order to limit the frequency band swept by the modulated signal solely to its useful portion. In fact, although SSB peak-limiting does not modify the spectrum of the modulated signal to any appreciable extent, a consequence of peak-limiting is nevertheless that it produces harmonics of said modulated signal. It is necessary to remove these harmonics prior to transmission. From a practical standpoint, the requisite complementary filter is identical with the first SSB filter.
Furthermore, it is recognized that there is now a real need, in industry, for the construction of SSB transmitters which are capable of transmitting at will on one of the two channels: the upper SSB channel or the lower SSB channel. This need is particularly critical in the field of radio transmission, in which channel switching is a common practice for overcoming certain disadvantages of transmission of radio waves in the atmosphere. In order to equip these two channels with the last-mentioned device, it is necessary to provide two pairs of filters, namely a pair of filters tuned to the lower-frequency band and a pair of filters tuned to the upper-frequency band. These filters are of the quartz type in particular, are produced one by one in accordance with a highly elaborate technology involving a large number of manual operations and are consequently very costly. For these reasons it is important to reduce the number of filters to a minimum.